[Fleet] Implement executePreCondition for packages state machine functionality

[criamico]

Phase 1 of #169147

Follow up of #175592.

In #178657 we introduced a new state machine. We need to add some more functionalities:

• Implement a function that get executed prior to each step in the state machine, similar to executePostTransition. It could be called executePreCondition for consistency.
• Add unit tests https://github.com/elastic/kibana/blob/5a09ec9a308a69acdab8c9470bfc447b9f11a645/x-pack/plugins/fleet/server/services/epm/packages/install_state_machine/state_machine.test.ts

Note that this change can be done without actually using it in x-pack/plugins/fleet/server/services/epm/packages/install_state_machine/_state_machine_package_install.ts, so it could be a first step for #175597. In fact the state machine is generic enough that we can use it modularly (i.e. we add an executePreCondition function but then we don't pass it to the actual state transitions).

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)